A static memory cell constructed from six transistors is commonly applied in memory designs to fulfill requirements for short access cycle times, high-frequency data rates, low power consumption, and excellent immunity from extreme environmental conditions.
With reference to FIG. 1A a six transistor (6-T) cell latches digital data in a memory cell latch loop formed by a pair of cross-coupled inverters in a prior art static memory cell diagram 101. A first complementary inverter is constructed from a first PMOS transistor 115 and a first NMOS transistor 125. A second complementary inverter is constructed from a second PMOS transistor 120 and a second NMOS transistor 130. A pair of access devices is used to connect and disconnect the memory cell latch loop from a bitline BL and a complementary bitline BL. The access devices are a third NMOS transistor 105 connected to the input of the first complementary inverter and a fourth NMOS transistor 110 connected to the input of the second complementary inverter. The access devices are enabled by a select signal on a wordline WL.
With reference to FIG. 1B a memory cell latch loop is represented as cross-coupled inverters 140, 145 and has two pairs of access devices forming two access ports in a prior art dual port memory cell diagram 102. Utilization of a memory array improves with simultaneous access to two different memory locations provided by dual memory ports. A first access port is formed by a first pair of NMOS transistors 110, 105 connecting from the memory cell latch loop to a first bitline BL1 and a first complementary bitline BL1. A first wordline WL1 enables the first pair of access devices. A second access port is formed by a second pair of NMOS transistors 165, 160 connecting from the memory cell latch loop to a second bitline BL2 and a second complementary bitline BL2. A second wordline WL2 enables the second pair of access devices.
With reference to FIG. 1C a row decoder 180 selects wordlines connected to memory cells within a memory cell array 170 in a prior art memory system diagram 103. A column decoder 185 selects the bitlines of the memory cells. Sense and write amplifiers 190 connect to the bitlines for reading and writing memory cells after a pair of bitlines is selected. A control block 175 connects to the row decoder 180, the column decoder 185, and a sense and write amplifier 190 to provide addresses and control signals for read and write operations.
U.S. Pat. No. 6,005,794 entitled “Static Memory with Low Power Write Port” to Sheffield et al. describes write port circuits of a static memory cell that include a first conditional conduction path between a first output of a latch and ground active if and only if both a wordline input and a write data true bitline input receive active signals. The write port circuit includes a second conditional conduction path between a second output of the latch and ground active if and only if both the wordline and a write data complement bitline receive active signals. The first and second conditional conduction paths may be formed by a series connection of the source-drain paths of two transistors. In each conditional conduction path the gate of a first transistor receives a corresponding column signal and the gate of a second transistor is connected to the wordline. The wordline transistors may be shared between bitline transistors of a single memory cell or of memory cells in plural contiguous adjacent columns. The memory cells may include a plurality of write ports with the write port circuit used for each port instance. While the '794 patent uses both a pulldown stack and a pullup stack to drive the read bitline, two PMOS transistors are required in each pullup stack replicated in every cell. The pullup stack replication increases an overall memory array size and complexity.
With reference to FIG. 2, a transfer curve 210 in a prior art inverter transfer characteristic diagram 200 transitions an equal potential line 205 at a point with an intercept of the Vin axis (abscissa) and Vout axis (ordinate) at about VDD/2. The equal potential line is a locus of points defined by an input voltage equaling an output voltage (Vout=Vin). The equal potential line is therefore a line at a 45 degree angle commencing from the origin. The transfer characteristic of the inverter is generic, having a low level input voltage Vin corresponding to a high level output voltage Vout and vice versa. In the case of a CMOS transistor implementation of the inverter, the beta ratios of the pull-up device and the pull-down device are matched to effect the transfer curve crossing of the equal potential line at about VDD/2.
More specifically, the pull-up and pull-down device are operating in their respective saturation regions at the operating point Vin=VDD/2. In order for the transfer curve transition of the equal potential line to occur at about
            V      DD        2    ,the following design considerations are followed as closely as possible: With the saturation current of the p-type pull-up device being
            I      dsp        =                  -                              β            p                    2                    ⁢                        (                                    V              in                        -                          V              DD                        -                          V              tp                                )                2              ,the saturation current of the n-type pull-down device being
            I      dsn        =                            β          n                2            ⁢                        (                                    V              in                        -                          V              tn                                )                2              ,and with a series connection of the pull-up and pull-down devices, then Idsp=−Idsn. Solving for Vin:
      V    in    =                    V        DD            +              V        tp            +                        V          tn                ⁢                                            β              n                                      β              p                                                  1      +                                    β            n                                β            p                              and setting βn=βp and Vtn=−Vtp, the result is
  Vin  =                    V        DD            2        .  